class_docstr: Interface for implementing a Trial Scheduler class.
  
  Note to Tune developers: If a new scheduler is added, please update
  `air/_internal/usage.py`.
code: def metric(self):
      return self._metric
  
imports: from typing import Dict, Optional, TYPE_CHECKING
  from ray.air._internal.usage import tag_scheduler
  from ray.tune.result import DEFAULT_METRIC
  from ray.tune.experiment import Trial
  from ray.util.annotations import DeveloperAPI, PublicAPI
label_desc: Use image node_id 2 for calling the TrialScheduler obj's underlying member method code with example usage: obj.metric() and returns: self